Table 2.

Summary and recommendations regarding antioxidant supplementation for people undertaking endurance training.

Antioxidant compound	Evidence summary – exercise-related effects
Anthocyanins	-Effects on oxidative stress and antioxidant enzymes are mixed and limited to systemic data only -Equivocal effects on endurance performance, VO2 max and post-exercise muscle recovery -May improve blood flow and vascular function, although this does not appear to translate to performance benefits -Insufficient supportive evidence to recommend to athletes
Astaxanthin	-Rodent studies show decreased muscle oxidative stress and improved endurance performance; although with possible hampering of training-induced Nrf2 signalling and antioxidant enzyme induction in muscle -Studies in humans are lacking and unclear with respect to effects on oxidative stress, antioxidant enzyme levels, skeletal muscle adaptations, endurance performance and post-exercise muscle recovery -Insufficient supportive evidence to recommend to athletes
Catechins	-Overall, effects on oxidative stress and antioxidant enzymes are equivocal; although cocoa flavanols can produce small beneficial effects on systemic markers of oxidative stress -Evidence not supportive of beneficial effects on endurance performance -Evidence equivocal on effects on skeletal muscle mitochondrial biogenesis and post exercise muscle recovery -Can improve vascular function, particularly in overweight/obese individuals – however, this does not appear to translate to improvements in exercise performance -Chronic supplementation may lower RER, increase fat oxidation, decrease carbohydrate oxidation and increase energy expenditure -Potential adverse effects on liver enzymes at high doses (EGCG >800 mg/day) and lack of clear safety threshold dose -Insufficient supportive evidence to recommend to athletes
Curcumin	-Rodent studies show improvements in skeletal muscle oxidative stress, mitochondrial biogenesis and endurance performance -Studies in humans are lacking and unclear with respect to effects on oxidative stress, antioxidant enzyme levels, skeletal muscle adaptations and endurance performance -Limited studies in humans are supportive of benefits on post-exercise muscle recovery, although further research is required to confirm this -Insufficient supportive evidence to recommend to athletes
Quercetin	-Minimal evidence of any beneficial effects on systemic markers of oxidative stress -No evidence currently in humans to suggest it will impact on mitochondrial biogenesis in muscle -May result in small beneficial effects on endurance performance, although this is mostly limited to untrained individuals -Effects on muscle recovery post muscle-damaging exercise are equivocal -Insufficient supportive evidence to recommend to athletes
Resveratrol	-Findings of rodent studies support improvements in skeletal muscle oxidative stress, antioxidant enzymes and exercise performance. However, evidence on these outcomes is limited and unclear in humans. -Limited evidence in humans suggests some hampering of skeletal muscle mitochondrial biogenesis and vascular function, but evidence is mixed -Future studies should use higher doses (i.e. >2g/day) for which systemic concentrations of resveratrol and its metabolites are much higher (However, there is an increased risk of adverse effects at high doses) -Insufficient supportive evidence to recommend to athletes
Vitamin C	-Has been shown to have mixed effects on systemic markers of exercise-induced oxidative stress and on post-exercise muscle recovery -No convincing evidence of endurance performance benefits -May improve vascular function with exercise, although this appears to be mostly limited to older individuals after acute infusion -While some rodent data suggests impairments in skeletal muscle mitochondrial biogenesis, this has not been explored in humans in the absence of other additional antioxidants -Insufficient supportive evidence to recommend to athletes
Alpha-lipoic acid	-Limited evidence is suggestive of benefits on systemic markers of oxidative stress and antioxidant enzymes -Evidence from animal studies shows mixed effects on skeletal muscle oxidative stress, antioxidant enzymes, mitochondrial biogenesis and endurance performance. However, there is a lack of studies in humans investigating these outcomes -Insufficient supportive evidence to recommend to athletes
Coenzyme Q10	-No convincing evidence of improvements in markers of oxidative stress, antioxidant enzymes or post-exercise muscle recovery -Unlikely to affect skeletal muscle mitochondrial biogenesis -May improve vascular function, particularly in individuals with heart disease; for whom improvements in VO2max may occur -Evidence largely mixed for effects on endurance performance -Insufficient supportive evidence to recommend to athletes
Vitamin A/β-carotene	-The limited available evidence does not support either vitamin A or β-carotene in improving markers of oxidative stress or improving endurance performance -Limited evidence from rodents shows a hampering of exercise-induced skeletal muscle antioxidant enzyme adaptations after supplementation with retinyl palmitate; however this has not been explored in humans -Insufficient supportive evidence to recommend to athletes
Vitamin E	-Studies mostly show improvements in oxidative stress markers -Studies mixed in terms of effects on endurance performance; with most beneficial effects shown in trained athletes at high altitude -Some rodent data indicates hampering of skeletal muscle adaptations to exercise; although effects of vitamin E alone on these outcomes (in the absence of other antioxidants) has not been explored in humans -Insufficient supportive evidence to recommend to athletes
Melatonin	-Studies show improvements in systemic markers of oxidative stress and antioxidant enzymes -Rodent studies are supportive of beneficial effects on skeletal muscle oxidative stress and antioxidant enzymes; however, these outcomes have not been explored in humans -Limited studies show acute supplementation is unable to improve time trial performance; but effects of chronic supplementation on performance in humans are lacking -Insufficient supportive evidence to recommend to athletes
N-acetylcysteine	-Evidence tends to favour an improvement in sustained exercise performance after acute and chronic NAC supplementation -Evidence from limited acute infusion studies is mixed with respect to effects on skeletal muscle antioxidant levels -Limited evidence suggests NAC might improve aspects of vascular function in older, but not younger participants -Adverse effects limit use of high doses of NAC (>70 mg/kg), although newer effervescent forms may overcome issues of taste and tolerance -WADA restrictions limit the use of infusions [442], which might limit the applicability of NAC infusion -Insufficient evidence to recommend to athletes. However, it may be beneficial and well tolerated with doses (<70 mg/kg) taken chronically over several days prior to an endurance event
Vitamin C + E	-A combined dose of 500 mg vitamin C + 400 IU vitamin E does not appear to have any adverse effects on skeletal muscle adaptations to endurance exercise training -A combined dose of 1000 mg vitamin C + 260–400 IU vitamin E has been found in some studies to hamper some markers of mitochondrial biogenesis and antioxidant enzyme induction -There appears to be no effect (beneficial or detrimental) of combined vitamin C + E on endurance exercise performance -Effects on post exercise muscle recovery are limited and equivocal -Insufficient supportive evidence to recommend to athletes. It might also be wise for athletes to avoid the combination of 1000 mg vitamin C + vitamin E during periods of heavy training in which skeletal muscle adaptations are occurring
Selenium	-Limited studies in humans have shown decreased exercise-related lipid peroxidation in overweight participants with low selenium levels -One study in humans showed hampering of markers of skeletal muscle mitochondrial biogenesis markers with exercise training; although evidence is limited and mixed overall -Limited studies show no beneficial effects on endurance performance -Insufficient supportive evidence to recommend to athletes
Zinc	-Limited evidence available shows some beneficial effects of zinc on systemic markers of exercise-induced oxidative stress -Evidence not supportive of effects on endurance performance, with only limited studies using zinc as the sole compound in supplements -Insufficient supportive evidence to recommend to athletes